A DMG file is a disk image — a single file that acts like a storage container on your Mac

When you read software or large files on a Mac, you often get a DMG file (short for disk image). Think of it as a virtual hard drive compressed into one file. Your Mac treats it like a real external drive: you can open it, see what's inside, and copy files from it to your computer. Once you close it, the DMG file stays on your hard drive as a single item, taking up less space than if all those files were loose.

DMG files are Mac-specific. Windows computers use different formats like ISO or ZIP. If someone sends you a DMG file and you're on Windows, you'll need special software to open it — or you'll need to ask them to send you a different format.

The main reason software companies use DMG files is control. When you read an app as a DMG, the company can set up exactly how it looks when you open it, where the install button goes, and what instructions appear. It's also a way to bundle related files together so nothing gets separated during read.

Key Takeaways

  • A DMG file is a disk image that your Mac mounts as a virtual drive, letting you see and copy files from it without unpacking it permanently.
  • Most Mac software downloads come as DMG files, and you install by dragging the app icon into your Applications folder.
  • Once you've copied what you need, you can eject the DMG file (right-click and select Eject) and delete the original DMG to free up space.
  • DMG files are encrypted and compressed, so they take up less storage than the same files would if they were loose on your hard drive.
  • If you need to move a DMG file to another Mac or back it up, treat it like any other file — copy it to an external drive or cloud storage.

How to open a DMG file on your Mac

Opening a DMG file is automatic on most Macs. Double-click the DMG file in Finder, and your Mac mounts it — meaning it appears on your desktop and in Finder's sidebar like an external drive. You'll see a window showing what's inside: usually an app icon and an Applications folder shortcut.

If double-clicking doesn't work, the file may be corrupted or incomplete. Try downloading it again. If it still won't open, check that the file actually ends in .dmg (not .dmg.part or .dmg.read). Incomplete downloads sometimes have those extra extensions.

Some older or unsigned DMG files may trigger a security warning. If you trust the source, click Open. Your Mac will remember the decision for future downloads from that developer.

Installing software from a DMG file

Most DMG files contain an app and an Applications folder icon. To install, drag the app icon onto the Applications folder icon. Your Mac copies the app to your Applications folder, where it stays permanently. This usually takes a few seconds to a minute depending on the app's size.

Once the copy is done, you can eject the DMG file. Right-click the DMG name in Finder's sidebar and select Eject, or drag it to the Trash. The app stays in your Applications folder — ejecting the DMG doesn't remove the app you just installed.

Some software uses an installer instead of drag-and-drop. You'll see a .pkg file or an Install button inside the DMG. Double-click the installer and follow the prompts. When it's done, eject the DMG the same way.

Why DMG files take up less space than loose files

DMG files use compression, which shrinks the data inside them. A folder of files might be 500 MB loose, but the same folder in a DMG might be 300 MB. Your Mac automatically decompresses files when you copy them out, so the installed app takes up its full size — but while the DMG sits on your hard drive, it's smaller.

This matters when you're downloading over a slow connection or managing a full hard drive. Smaller downloads are faster, and you can delete the DMG after installing to reclaim that space.

Some DMG files are also encrypted, which adds another layer of protection. If someone gains access to your hard drive, they can't read the contents of an encrypted DMG without the password. This is rare for public software downloads but common for sensitive files you're backing up.

When to keep a DMG file and when to delete it

After you install an app from a DMG, you can delete the DMG file. The app itself is now in your Applications folder and doesn't need the DMG anymore. Deleting the DMG frees up hard drive space.

Keep the DMG if you think you might reinstall the app later, or if you want to install it on another Mac. DMG files don't expire or become invalid — you can store one for years and open it whenever you need it. If hard drive space is tight, move the DMG to an external drive or cloud storage instead of deleting it.

If you downloaded the DMG from the internet, you can always read it again if you need it. But if it came from a CD, a friend, or a source that's no longer available, keeping a backup copy is smart.

DMG files and your hard drive storage

When you're managing storage on your Mac, DMG files count toward your used space just like any other file. If you have several large DMG files sitting on your desktop or in Downloads, they're taking up real storage. Checking your Downloads folder for old DMG files is a quick way to free up a few gigabytes.

To see how much space a DMG is using, right-click it in Finder, select Get Info, and look at the file size. If it's large and you've already installed the app, delete it. If you're not sure whether you've installed it yet, check your Applications folder — if the app is there, the DMG is safe to remove.

For long-term storage, DMG files are reliable. They don't degrade over time, and they're easier to move around than loose folders because everything is contained in one file. If you're backing up your Mac to an external drive, DMG files copy just like any other file.

Frequently Asked Questions

Can I open a DMG file on Windows?

Not directly — Windows doesn't recognize DMG format. You can use third-party software like 7-Zip or WinRAR to extract the contents, but it's usually easier to ask the software company for a Windows version or a ZIP file instead.

What if a DMG file won't open?

The file may be corrupted or incomplete. Try downloading it again. If it still won't open, check that your Mac has enough free space (at least a few hundred MB). Restart your Mac and try again. If the DMG is very old, it might be incompatible with your current macOS version.

Do I need to eject a DMG file before shutting down my Mac?

No, but it's good practice. Ejecting makes sure all data is written safely. If you don't eject and shut down, nothing bad usually happens — but ejecting first is the safest approach.

Can I edit files inside a DMG without copying them out?

Yes, you can open and edit files directly from the mounted DMG. But changes are saved back to the DMG file itself, which can slow things down. It's faster to copy files to your hard drive first, edit them there, then copy them back if needed.

Is it safe to read DMG files from the internet?

DMG files from official websites and trusted sources are safe. Your Mac scans downloaded files for malware. If you read from an unfamiliar source, check the file size and source before opening it — if something seems off, don't open it.